Studying the mechanisms of regulation of the neuronal fate is of importance to understand the determination, differentiation, and maintenance of neurons. Second, identification of extracellular and intracellular regulators that are important in determining the neuronal phenotype, has attracted considerable interest because of the possible therapeutic importance in treatment of several neurodegenerative diseases. The role of extracellular signals in determining the diversity of vertebrate nervous system has been studied extensively. The most well characterised of the secreted factors involved in the control of developing and adult nervous system are the nerve growth factor (NGF) and the glial-cell derived neurotrophic factor (GDNF) families of neurotrophic factors (reviewed in Bibel and Barde, 2000, Genes Dev 14:2919-2937; Airaksinen et al 1999, Mol Cell Neurosci 13:313-325; Airaksinen and Saarma 2002, Nat Rev Neurosci 2002 3:383-394). These neurotrophic factors promote survival, differentiation and maintenance of specific neuronal populations in vertebrates. Later it has been shown that they have other important functions, including regulation of activity-dependent synaptic plasticity, stimulation of neurite outgrowth, and protection and repair of neurons during tissue injury.